The present invention relates to a thermal dot type printing apparatus in which overheating of the thermal print head is prevented.
There is known a thermal dot type printing apparatus in which a carriage is moved at a constant speed and current is allowed to pass through each heating element of a thermal print head mounted on the carriage for dot printing, thereby printing characters corresponding to printing data. In this type of printing apparatus, when the total number of heating elements which are energized in a unit time increases, the temperatures of the thermal print head and of the parts near the thermal print head also increase. When these temperatures increase and become higher than a preset limit value, the size of the dot which is printed becomes large, so that the printed characters become unclear and in the worst case, there is a risk that a print ribbon will melt due to the heat and become welded and deposited on a portion of the print head or on a printing paper.
To avoid such a problem, in a conventional thermal dot type printing apparatus, a temperature detecting element such as a thermistor is attached to the thermal print head and the amount of energy from a voltage which is applied to, or from a current which is allowed to flow through each heating element is changed in accordance with the temperature of the portion of the thermal print head.
However, the thermal dot type printing apparatus in which the temperature detecting element is attached to the thermal print head has the following problem. Namely, a temperature detecting element and a temperature detecting circuit which is connected to the temperature detecting element are costly; therefore, there is a problem such that the manufacturing cost of the overall thermal dot type printing apparatus and the cost of the printing apparatus increase.
On the other hand, if the printing time interval is set long for prevention of the increase in temperature of the thermal print head, a problem, arises that the printing speed contrarily slows down.